1. Field of the Invention
This invention relates to an apparatus for stabilizing frequency of a light source which is utilized in wavelength division multiple (Wavelength Division Multiplex. Hereinafter, referred to as WDM) optical communication.
2. Description of Related Art
In the optical communication field, it is required to enlarge transmission capacity. In order to enlarge transmission capacity, the conventional apparatus multiplex electrical signal (frequency division multiplex or time division multiplex), or increases numbers of optical fibers. Thus, total amount of transmission is enlarged.
However, such demand for large transmission capacity grows rapidly. For this reason, development of WDM optical communicative technique is advanced in recent years. For this WDM optical communication, a WDM light source which generates an optical wavelength division multiplexed signal is used. Outputs of the WDM light sources is inputted into an optical fiber. Thus, a plurality of transmission paths can be configured in an optical fiber, and it becomes possible to meet the demand. Here, the technique for increasing higher stability for frequency of WDM light source should be indispensable.
For example, a technique disclosed in, Sakamoto et al, xe2x80x9cCharacteristics of optical senders"" frequency stabilizer employing a Mach-Zehnder filter under real environmentxe2x80x9d, The Institutes of Electronics, Information and Communication Engineers B-10-216, 1997, has been proposed. In this document, frequency of WDM light source is adjusted to one of peak frequency of mach-zehnder filter.
By the way, when a mach-zehnder filter is used, an attention should be paid on the following in order to make frequency of WDM light source be stable:
1. A mach-zehnder filter has characteristics in which pass band and stop band repeatedly appears with equal band. But in actual apparatus, center frequency of the bands may sometimes slip from designed frequency by an error on manufacture or a polarization dependency. Therefore, frequencies of the plural light sources may not match the center frequencies of the each band of the filter.
2. Because of the above-mentioned characteristics of the mach-zehnder filter, frequency of each light source can not be chosen freely.
3. Frequency of WDM light source needs to be led to a center frequency of the mach-zehnder filter. For this reason, voltage is applied at the filter, and thus the characteristics of the filter should be dizzered.
Accordingly, it is the object of this invention to provide WDM light source which does not use the Mach-zehnder filter.
It is another object of this invention to provide WDM light source with frequency of each light source becomes more stable, and thus more efficient and precise control can be achieved.
It is further object of this invention to provide WDM light source with reduced circuit scale.
According to the first aspect of the invention, the WDM light source comprises a light source which generates an optical signal for an optical channel. The optical signal is intensity modulated with modulating signals, the signal is assigned to each channel. The modulated optical signal is applied to a dispersion generating circuit, at which dispersion occurs on the optical signal, according to the wavelength of the signal. The WDM light source further comprises a phase difference detector which detects phase difference between the optical signal input to the -dispersion generating circuit and the optical signal output from the dispersion generating circuit. According to the detected phase difference, the WDM light source generates control signal which corresponds to delay time. The control signal is fed back to the light source thus the wavelength of the light source is controlled. As a result, the wavelength is stabilized within the predetermined range.
According to the another aspect of the invention, the WDM light source comprises a plurality of light sources, each generates an optical signal for an optical channel. The optical signals are intensity modulated with modulating signals, the signal is assigned to each channel. The optical signals are duplexed and applied to a dispersion generating circuit, at which dispersion occurs on the optical signal, according to the wavelength. The control signal is generated for each light source according to the each detected phase difference, and the control signal is fed back to the light source thus the wavelength of the light source is controlled. As a result, the wavelength is stabilized within the predetermined range.
According to the yet another aspect of the invention, the WDM light source comprises a plurality of light sources, each generates an optical signal for an optical channel. The WDM light source further comprises switching element that selects one of the optical signal among the plurality of optical signals. The selected optical signal is fed to a modulator which intensity modulates the optical signal with modulating signals, the signal is assigned to each channel. The modulated optical signals applied to a dispersion generating circuit, at which dispersion occurs on the optical signal, according to the wavelength. The control signal is generated for each light source according to the detected phase difference, and the control signal is fed back to the light source thus the wavelength of the light source is controlled. As a result, the wavelength is stabilized within the predetermined range.